Adding a more polished Network class.

Network.py provides a more polished version of the Network class from
backprop2.py. Improvements are:

* Better documentation.
* Saving and loading.
* List-like operators (splicing, concatonating).

(The list-like operators are convenient for pretraining work.)

This is intended for an experimental branch, but github won't let me propose a pull request to a non-existant branch.